a) Field of the Invention
The present invention relates to a charged particle beam exposure method, and more particularly, to a charged particle beam exposure method which can be compensate proximity effect.
b) Description of the Related Art
In a charged particle beam exposure, charged particles are injected to a resist film to expose the resist film. Charged particles incident on the resist film undergo multiple scattering as they proceed.
In a case wherein a resist film is formed on a silicon substrate and a charged particle beam exposure is performed thereon, the charged particles incident on the resist film are subjected to forward scattering as they proceed within the resist film, and proceed to the silicon substrate. The charged particles which have reached the substrate may proceed from the substrate to the resist film again by large angle scattering (backward scattering) in the substrate.
An exposure pattern will have a parasitic pattern formed around a designed exposure pattern due to such forward scattering and backward scattering as mentioned above. When the exposure patterns are densely located, parasitic patterns due to scattering around each of the pattern are superposed so that an intensity of exposure outside the exposure patterns may exceed a threshold level of development.
As a result, there occurs such phenomenon that patterns having a larger size than a designed size are obtained. This phenomenon is called a proximity effect since it is remarkable when the patterns to be exposed are densely located in proximity to one another.
FIGS. 6A, 6B and 6C are diagrams for illustrating the proximity effect. FIGS. 6A and 6B are graphs showing trajectories of electrons when a resist film 2 formed of PMMA is disposed on a silicon substrate 1 and electron beam is irradiated thereon from upward. FIG. 6A shows trajectories of electrons when they are irradiated with an acceleration energy of 10 kV, and FIG. 6B shows trajectories for a case with an acceleration energy of 20 kV.
Each of the graphs of FIGS. 6A and 6B is derived from a simulation by Monte Carlo method of trajectories of 100 electrons. Abscissae of the graphs refer to a distance of the resist film 2 from an irradiating position of electrons in micron, and ordinates of the graphs refer to a depth from the surface of the resist film 2 in micron.
As is apparent from the graphs, electrons irradiated on the resist film 2 reach, because of forward scattering and backward scattering, to a depth of about 2 .mu.m in a case in which acceleration voltage is 10 kV, and about 4 .mu.m in a case in which acceleration voltage is 20 kV.
FIG. 6C is a diagram showing schematically a distribution of exposure intensity due to such electron beam exposure. The intensity is strong at portions of exposure patterns P1, P2, and P3. Tail portions T1, T2, and T3 are formed therearound by forward scattering and backward scattering. The intensity at the tail portion depends on an area of the pattern. It is strong when the pattern is wide and is weak when the pattern is narrow.
With a given acceleration energy of the charged particle beam, extension of tail portion T due to scattering will be approximately constant. The intensity of the tail portion which extends around each pattern reduces as a distance from the pattern increases.
Here, the tail portions T1 and T2 of the patterns P1 and P2, respectively, as shown in the figure, are superposed mutually in the intermediate region, giving a sum effect on the resist film. Thus, at portions with dense patterns, tails from each pattern are superposed, and, thereby, may exceed a threshold value of development.
FIG. 6D is a diagram to illustrate a proximity effect caused by a superposition of tail portions which originate from forward scattering and backward scattering. In a case of exposing rectangular patterns P5 and P6 positioned in parallel, as shown on the left hand side of FIG. 6D, when a charged particle beam exposure is performed on the exposure patterns which are designed according to desired patterns, resulting patterns of the exposure will become as shown on the right hand side of FIG. 6D.
Namely, at a central part of a gap between the patterns P5 and P6, tail portions of each part of the patterns superpose thickly, thereby, to thicken an exposed width of the patterns than designed. Thus a desired shape of pattern cannot be obtained when a proximity effect occurs.
Means for obtaining a desired shape of pattern by compensating proximity effect beforehand is called a proximity effect correction. FIGS. 7A to 7D show diagrams illustrating prior art techniques of compensating proximity effect.
FIG. 7A shows schematically a case wherein no compensation of proximity effect is given and a proximity effect is generated between two subject patterns. When rectangular patterns P5 and P6 are exposed, the patterns would have an thickened portions at their centers and patterns P5a and P6a with thickened portions may be connected at the central portion P7. Methods for compensating such proximity effect are described hereunder.
FIG. 7B shows a method of compensating proximity effect by changing an irradiation intensity of charged particle beam. In exposing the patterns P5 and P6, proximity effect as shown in FIG. 7A occurs, if the whole area within the patterns is exposured with a uniform intensity.
To compensate the proximity effect, irradiation intensity is reduced at portions in the vicinity of other patterns. For example, as shown in the diagram, sampling point X is taken at a center of each side of the patterns P5 and P6 facing to each other. An amount of irradiation of charged particle beam including effect of scattering from patterns in the vicinity is calculated at each of the sampling points. The amount of irradiation on the adjacent portions P8 and P9 is adjusted to obtain a pre-determined exposure amount.
By reducing the amount of irradiation of charged particle beam on the adjacent portions P8 and P9, scattering of the charged particle beam at a region P7 between the patterns is reduced, and the proximity effect is compensated.
FIG. 7C is a diagram for illustrating a pattern elimination method, which is another method of compensating proximity effect. When patterns P5 and P6 are exposured as designed, each pattern becomes larger than the designed size. Thus, a portion of the pattern is curtailed in advance to incorporate the pattern size increment by scattering, thereby obtaining a pre-determined size of resultant exposure pattern.
For example, a sampling point X is taken at each center of side of the patterns P5 and P6 facing to each other. An amount of irradiation of charged particle beam including effect of scattering from patterns in the vicinity thereof is calculated at each of the sampling points. A portion of each of the adjacent regions P10 and P11 is curtailed. Thus, exposed patterns will form patterns P5 and P6 which have desired widths in stead of wider patterns without compensation.
However, since both of the pattern elimination method and the irradiation intensity reducing method need compensation calculation for all the patterns to be exposed, a time required for performing compensation calculations increases drastically as the number of patterns increases. Further, since both of the compensation require representative points for obtaining compensation value of exposure intensity thereat. If the number of representative points is not enough, it becomes impossible to fully compensate distortions due to proximity effect. A larger number of representative points will cause an increase of the time of compensation calculation.
Moreover, for a block exposure method which copies a repetition of complex patterns formed on a mask collectively, the pattern eliminating method and the irradiation intensity adjusting method, which compensate for each of the patterns, are extremely difficult to perform, practically.
FIG. 7D shows a ghost exposure method which is another method of compensating proximity effect. The ghost exposure method uses a main pattern and a supplementary pattern which is a reversal pattern of black and white of the main pattern for forming an exposure pattern.
After an exposure using the main pattern, an exposure using the supplementary pattern is performed with an intensity corresponding to an intensity of backward scattering. Such an additional exposure will provide a uniform exposure outside of the main pattern, and development of only the main pattern can be provided by adjusting the level of development.
The ghost exposure solves problems of insufficient compensation of proximity effect and increase of time for compensation calculation of proximity effect. However, it is necessary to form a reversal pattern of the main pattern, and, further, exposure time is elongated by an exposure of a complicated reversal pattern.
These methods of compensating proximity effect premise that, in determining conditions of exposure, a substrate to be exposed is formed of a uniform material.
In such a uniform material, spreading of an incident electron beam due to forward scattering and backward scattering can be approximated as: EQU F(r)=exp(-r.sup.2 /A.sup.2)+B exp(-r.sup.2 /C.sup.2) (1),
where the first term on the right hand side represents forward scattering, and the second term represents backward scattering. In the formula (1), the normalization constant is neglected. A parameter B represents an intensity ratio of the backward scattering with respect to the forward scattering. Parameters A and C represent extension of Gaussian distribution of electron beam due to scattering. And r denotes a distance from a point of irradiation of the electron beam.
If exposure of a substrate is to be done taking various patterns into consideration without premising a uniform material, the constants B and C in the formula (1) have to be considered as a function of location. Then, the number of data, as of exposure data, to be taken into account at the time of calculation increases extremely so that a very long time would be needed in treatment. Consequently, lower level patterns which could have been formed on the lower level of the substrate has not been considered heretofore.
Practical substrates have often experienced with various semiconductor processes, and thus have various thin-layers as of SiO.sub.2, Si.sub.3 N.sub.4, Al, Ti, W, or the like formed thereon. Such substrates are far from being uniform. However, for exposing a resist layer on such a substrate formed of various thin layers, conditions of exposure have not been determined upon considering each underlying layer, but have been determined, for convenience sake, with a premise that the substrate is formed of a material of uniform quality. Thus, due to non-uniformity of an intensity of backward scattering, there occurred cases that proximity effect may have been compensated sufficiently at some portions but may not have been compensated sufficiently at other portions. In particular, when a wiring of a material having a large atomic number such as tungsten, which has a large cross section of scattering for electrons, is patterned underneath, backward scattering intensity at a portion covering the wiring becomes abnormally large compared to other portions. Accordingly, only at this portion, proximity effect cannot be compensated sufficiently, thereby producing resolution defects thereat.
FIGS. 8A, 8B, 9A, 9B, 10A, and 10B show a case of selective exposure on a resist film covering a lower level tungsten pattern. In FIGS. 8A and 8B, a lower level pattern 52 of W is formed. In FIGS. 9A and 9B, an upper level layer 53 is deposited on the lower level pattern, and a resist film 54 is applied on the upper level layer 53. Charged particle beam is irradiated on the resist film 54 according to a data of upper level patterns to expose the resist film 54. FIGS. 10A and 10B show a resultant substrate. First, the resist film 54 is developed. The upper level layer 53 is etched using the developed resist film 54 as a mask. Then, the resist film 54 is removed. At a portion where the upper level layer 53 covers the lower level layer 52, the upper level layer 53 has a wider dimension, and hence a resolution defect thereat.